The invention concerns a solid phase with at least one test area to detect an analyte which additionally comprises at least one control area to detect interferences. Furthermore the invention concerns a method for the detection of one or several analytes using a solid phase according to the invention in which interfering reactions can be detected and corrected if necessary.
When analytes are detected by binding assays interferences in the detection reaction occur with some samples which lead to false test results. This phenomenon is usually referred to as a matrix effect of the sample. The presence of an interference cannot in general be indicated in conventional test formats. It is attempted to prevent matrix effects of the various samples or to reduce them as far as possible by elaborate optimization of the solid phase, test buffers and detection reagent. However, such a reduction of interference in detection methods is complicated and expensive. Moreover one cannot completely rule out the possibility that an adequate reduction of interference does not occur for certain samples despite test optimization since it is unfortunately not possible to completely suppress matrix effects. In addition new interferences which were unknown during the development of the detection method can occur which also lead to false results. Consequently there is a problem that in the known detection methods false test results can be obtained without the user being aware of this. This circumstance is particularly tragic in the case of qualitative tests for the detection of an infectious disease. Thus for example a false positive sample resulting from matrix problems has considerable consequences for a HIV test.
U.S. Pat. No. 4,558,013 describes a test strip which contains a non-defined uncoated negative control region in addition to test regions coated with specific test reagents. The value measured in the test region is corrected by subtracting the unspecific binding in the control region. However, such a procedure can only partially correct for interferences since the unspecific binding of interfering components to the test region usually differs considerably from the binding of interfering components to the uncoated control region.
U.S. Pat. No. 5,356,785 describes a solid phase with several test areas which each contain different amounts of a solid phase receptor for the detection of an analyte. The solid phase additionally contains a reference area which generates a detectable signal of known intensity with the test reagent. Control areas to determine unspecific interactions between the sample and the solid phase are not disclosed.
U.S. Pat. No. 4,916,056 (Brown III et al.) describes a solid phase for the qualitative or quantitative determination of an analyte, in particular of an antigen, antibody or DNA segment in a sample. The solid phase contains, in addition to a test area, a reference area which yields a positive signal in the test and a non-defined negative control area which contains the positive reference area and the test area. A disadvantage of this device is that the surface of the uncoated control region differs too greatly from the test area to achieve an effective correction for interferences.
Hence an object of the invention was to provide devices and methods for the detection of analytes which enable a direct indication of the presence and optionally the type of interferences such that these interferences can be taken into account when evaluating the test results.
This object is achieved according to the invention by a solid phase having at least one defined test area for the detection of an analyte in a sample which is characterized in that the solid phase additionally comprises at least one defined control area for the detection of interferences. In this connection the term xe2x80x9cdefined test areasxe2x80x9d on a solid phase is understood to mean that the test areas comprise defined regions of the solid phase which are preferably spatially separated from other test areas by inert regions. The defined test areas preferably have a diameter of 10 xcexcm to 1 cm and particularly preferably 10 xcexcm to 5 mm. Miniaturized test areas with a diameter of 10 xcexcm to 2 mm are most preferred. Solid phases with several test areas are preferred which are also referred to as array systems. Such array systems are for example described in Ekins and Chu (Clin. Chem. 37 (1995), 1955-1967) and in U.S. Pat. Nos. 5,432,099, 5,516,635 and 5,126,276. An advantage of array systems is that several analyte and control determinations can be carried out simultaneously on one sample. The use of control areas to detect unspecific binding and/or interfering samples can considerably improve the reliability of the results especially with miniaturized array test systems.
In this connection the detection of interferences and unspecific binding in qualitative tests and in particular in those which have stringent requirements for specificity such as tests for infections (e.g. HIV) are of particular interest. The indication of an interference and correction of the test result enables false-positive results to be considerably reduced thus leading to an enormous improvement of specificity.
The solid phase according to the invention is any conventional support for detection methods, preferably a non-porous support e.g. a support with a plastic, glass, metal or metal oxide surface. Porous supports such as test strips are also suitable. Spatially discrete regions (test areas) are located on this support. Immobilized solid phase receptors are applied to these test areas. The solid phase receptors are immobilized by known methods e.g. by direct adsorptive binding, by covalent coupling or by coupling via high affinity binding pairs e.g. streptavidin/biotin, antigen/antibody or sugar/lectin. The presence or/and the amount of the analyte in a sample can be determined by specific binding of components from the detection medium e.g. of the analyte to be determined or of an analyte analogue to the solid phase receptor.
The detection of the analyte and the presence of interfering reactions is achieved in the method according to the invention in a known manner by using suitable marker groups e.g. fluorescent marker groups. Alternatively with suitable solid phases it is possible to also detect the interaction of components of the detection medium with the test and control areas by determining the layer thickness of the respective area e.g. by plasmon resonance spectroscopy.
With array systems in which several analytes from a sample are detected simultaneously, it is preferable to use a xe2x80x9cuniversalxe2x80x9d marker group which enables a simultaneous detection of several different analytes to different test areas. An example of such universal marker groups are marker groups which carry a receptor that can specifically interact (e.g. by means of a high-affinity binding pair such as antibody/antigen or streptavidin/biotin etc.) with a complementary receptor on a test reagent e.g. a soluble receptor for an analyte to be determined or for an analyte analogue.
The application of such a universal marker group is exemplified in FIG. 1. In this case a fluorescent latex bead which is coupled with an anti-digoxigenin antibody ( less than Dig greater than  label) is used for three different test formats on a single solid phase i.e. to determine HIV antibodies, HBs antigen and anti-HBc antibodies. In the case of the anti-HIV antibody determination an immobilized HIV antigen and a digoxigenylated soluble HIV antigen are used which form an immobilized immune complex with the anti-HIV antibodies to be detected. The marker group can bind to the digoxigenin groups present on this immune complex. For the determination of HBs antigen, an immobilized antibody and a digoxigenylated soluble antibody that can interact with the marker group are used in a corresponding manner. A competitive test format is used for the anti-HBc determination in which anti-HBc antibodies present in the sample compete with a digoxigenylated anti-HBc antibody for immobilized HBc antigen. The quantity of marker group bound to the test area is inversely proportional to the anti-HBc concentration in the sample.
Defined test and control areas can additionally contain a detectable and analyte-unspecific marker group which can be detected concurrently with the analyte-specific marker group and does not interfere with it, in order to differentiate them from inert regions of the solid phase. An example of such an analyte-unspecific marker group is a fluorescent marker group which fluoresces at a wavelength which is different from the fluorescent wavelength of an analyte-specific marker group. The analyte-unspecific marker group is preferably immobilizedxe2x80x94like the solid phase receptorxe2x80x94via a high affinity binding pair e.g. streptavidin/biotin.
The solid phase according to the invention can be used in any detection methods e.g. in immunoassays, nucleic acid hybridization assays, sugar-lectin assays and similar methods.
The solid phase according to the invention enables a detection of interfering reactions to obtain reliable test results even when the measures known from the prior art for interference reduction are not adequate for certain samples. The control areas not only enable a qualitative detection of interfering reactions but also in many cases enable a quantitative correction for the interference.
The solid phase can comprise several, in particular different control areas for the detection of interfering reactions in order to detect different interferences. In this manner it is possible to specifically detect different types of interference. It is particularly advisable to apply a series of control areas that are suitable for the detection of frequently occurring or/and particularly relevant interfering components for the respective test.
Interference of test procedures can in general be the result of undesired, non-analyte-specific interactions of substances on the test area with components of the detection medium. These substances on the test area may be in particular components of the solid phase, parts of the solid phase receptor and other reagents located on the surface of the solid phase. The interfering components of the detection medium mainly come from the sample (matrix effect) and in some cases lead to unspecific binding of test reagents e.g. of the detection reagent to the solid phase and result in a falsification of the measured signal. Thus interfering interactions between the test area and sample components, test reagents, reaction products or complexes of sample components and test reagents may occur.
The solid phase according to the invention preferably comprises control areas to detect interferences that are due to an undesired binding of components of the detection medium to the specific solid phase receptor for the analyte. Non-analyte interfering components are frequently present in samples such as antibodies or antigens which have a tendency for an increased unspecific binding to the solid phase receptor and in this manner lead to erroneous test results. Hence a control area is particularly advantageous which comprises a non-analyte specific solid phase receptor which, with the exception of the region that can specifically bind to the analyte, is completely identical to the solid phase receptor in the test areas.
If the solid phase receptor is for example an antibody or an antibody fragment, a control area is used which contains an unspecific antibody or an unspecific antibody fragment of the same species, preferably of the same class and particularly preferably of the same subclass as that of the solid phase receptor of the test area. If the solid phase receptor is for example an antigen e.g. a peptide or a polypeptide, a control area is used which contains a mutated xe2x80x9cantigenxe2x80x9d which differs from an immunologically reactive antigen by modifications e.g. by modifying the smallest possible number of amino acids in the region of immunogenic epitopes. These amino acid modifications can comprise insertions, deletions and preferably substitutions of natural amino acids by other natural amino acids or non-natural amino acid derivatives e.g. D-amino acids. If the solid phase receptor is for example a nucleic acid, a control area is used which contains a xe2x80x9cmutatedxe2x80x9d nucleic acid which differs from the nucleic acid immobilized on the test area by modifications in the nucleotide sequence for example by a base substitution within the sequence responsible for the recognition of a target nucleic acid, preferably in the middle thereof. A mutein of the solid phase receptor is most preferably used which, with the exception of the analyte-specific antigen binding site, is completely identical to the solid phase antibody in the test areas.
It is also preferable that the solid phase receptor applied to the control area has been subjected to identical treatment steps e.g. derivatizations as the solid phase receptor applied to the test area. Thus for examplexe2x80x94in the case of a biotinylated solid phase receptorxe2x80x94the number of biotin molecules coupled to the solid phase receptor should be the same in the test area and in the control area. In addition the solid phase receptors in the test area and in the control area should have been subjected to an identical coupling chemistry. In addition identical linkers should also have been used. A solid phase receptor suitable for a control area should not be able to specifically bind to the analyte but preferably comprises all other regions and thus binding sites of the solid phase receptor of the test area. Consequently the unspecific binding of interfering components to the respective solid phase receptor of the test area and control area is essentially identical so that it is possible to quantitatively correct the measured value of the test area on the basis of the measured value of the control area.
The solid phase additionally comprises at least one control area to detect interferences which are caused by the reaction of other immobilized reagents in the test areas with non-analyte components of the sample. As a result interfering components are especially detected that react specifically or/and unspecifically with components of the loading solution used to apply the solid phase receptor. Such a control area can for example contain reagents of the loading solution used in a test system such as buffers, immobilization reagents such as streptavidin, biotinylated substances e.g. biotinylated fluorescent markers, non-analyte specific antibodies etc., blocking reagents or linkers.
Rheumatoid factors often interfere with tests. Hence it is preferable to set up at least one control area on the solid phase to detect rheumatoid factors. Rheumatoid factors are usually IgM molecules but rarely also IgG, IgA and IgE molecules which react with the Fc part of antibodies and interfere with the test if they for example cross-react with the antibody immobilized on the test area or/and a soluble detection antibody, e.g. by cross linking the antibody bound to the solid phase with a labelled detection antibody which results in an unspecifically increased signal. For such a control area an unspecific IgG molecule, preferably the Fcy part of a human IgG molecule, is applied to the solid phase. Then the rheumatoid factor does not only bind to the test area during the test but also to the control area and thus indicates the interference.
A further interference which occurs relatively frequently is caused by foreign-species-specific antibodies in the sample i.e. antibodies which are directed against antibodies of foreign species e.g. human anti-mouse antibodies (HAMA). Thus the solid phase according to the invention also preferably comprises at least one control area to test for foreign-species-specific antibodies. Foreign-species-specific interfering components e.g. HAMA lead for example in a double antibody sandwich assay to a cross linking of the solid phase antibody with the detection antibody and consequently to an unspecifically increased signal. An unspecific antibody of the same species as the test antibody is preferably used for a suitable control area. A HAMA interfering component which may be present in the sample binds to the antibody applied to the control area which indicates the interfering reaction.
Instead of a solid phase receptor that is identical with the detection reagent in the test area apart from the specific analyte-bindable region, the complex formed during the detection reaction e.g. solid phase antibody-analyte-detection antibody (without label) can also be applied to a control area so that a specific reaction is no longer possible although the unspecific binding sites are almost identical with those of the test area.
Interfering components in the serum are also known which are directed against neo-epitopes of an antibody fragment. Such neo-epitopes are formed for example during the F(abxe2x80x2)2 cleavage of an IgG molecule. In this case it is preferable to provide a control area which contains a fragment of an unspecific antibody which has been produced by the same method (cleavage conditions and cleavage protein) as the antibody fragments of the test area. The interfering components bind to these unspecific antibody fragments and can therefore be detected.
In addition it is also possible using suitable control areas to detect interfering components which may be present in a sample e.g. interfering antibodies that are directed against immobilization reagents on the test areas such as streptavidin. For this purpose streptavidin (SA) is applied to a control area and labelled SA is added to the detection reagent. If anti-SA antibodies are present in the sample, a sandwich complex is formed and the antibody is specifically detected.
The solid phase according to the invention can further contain a control area to detect the total IgE content. Such a control area is particularly recommended for allergy tests. When specific IgEs are determined in allergy diagnostics samples with a high total IgE content often interfere with the specific detection reaction of a certain IgE. The total IgE content can be determined in parallel to the detection method with the aid of a control area on which anti-IgE antibodies have been applied.
Finally control areas can also be provided to detect interferences that are caused by reactions of components of the detection medium with the solid phase support. For this purpose all components of the solid phase support such as the support material (e.g. polystyrene), plasticizer, functional groups etc. can be applied to a control area.
In addition control areas to determine a cut-off value can be used for certain test procedures e.g. in qualitative or semi-quantitative tests for infectious diseases, allergies etc. The xe2x80x9ccut-offxe2x80x9d value is a threshold value that is set for test procedures in order to differentiate between positive and negative values. Such a xe2x80x9ccut-offxe2x80x9d value is of particular importance for test procedures which relate to infectious diseases. One possibility is to use a xe2x80x9cnegativexe2x80x9d control area which can contain a loading solution without the test reagent or a mutein of the test reagent. A major advantage is that a sample-specific value for the unspecific binding is determined for each sample and thus an improved specificity of the test is obtained. In this manner a separate negative control can be omitted.
FIG. 2 shows the effect of a control area used in a test in which a cut-off value is employed. With a large number of samples a certain number of false-negative and false-positive results are obtained (left side) with a conventional test procedure. The use of control areas according to the invention (right side) can selectively reduce the signals from negative samples which leads to a reduction of the cut-off value. In this way a positive/negative differentiation can be made with a considerably reduced probability of error.
A positive control can also be simulated by a reference area which contains the analyte or an analyte-like reagent. A major advantage is that a combination of a negative control area with a positive reference area enables a sample-specific cut-off value to be determined for each individual measurement. An advantage of this is that an indirect calibration is not necessary and an improved test specificity is achieved.
It is of course possible to provide other or/and additional control areas in addition to the aforementioned preferred control areas depending on the test procedure in order to detect interfering components that are presumed to be or are present in a sample.
The control areas provided according to the invention not only enable a qualitative detection of interfering reactions but often also a quantitative correction for the interference. With a suitable selection of test conditions the unspecific binding of non-analyte components occurring in the test areas is accurately reflected in certain control areas. Thus the measured value in a test area can be simply corrected to obtain a correct and unbiased result. Even if the unspecific binding in the test area and the control area are not identical, the measured signal can be corrected in qualitative tests since in this case only a statement of xe2x80x9cpositivexe2x80x9d or xe2x80x9cnegativexe2x80x9d is necessary.
An additional advantage is that the solid phase according to the invention enables the presence of several interfering components to be detected separately and it is possible to determine the type of interference. The user recognizes immediately that the result may be falsified due to the presence of one or several interfering components. The user can then either correct the measured results on the basis of the data that were determined, repeat the measurement with another test format or pretreat the sample in a suitable manner e.g. by separating the interfering components.
The solid phase according to the invention can be used to detect an analyte in a sample e.g. in a body fluid such as blood, serum, plasma, saliva etc. Solid phases with several test areas, i.e. array systems, for the simultaneous detection of several analytes are preferably used. At least one control area is preferably used for each test area in such array systems. The solid phases according to the invention can be used in all known heterogeneous test procedures especially in immunoassays and nucleic acid hybridization assays.
Hence a further subject matter of the invention is a method for the detection of an analyte using a solid phase with at least one defined test area which in addition comprises at least one control area to detect interfering reactions. The method according to the invention preferably includes the use of the control areas for the quantitative correction of interferences.
Finally the invention also concerns the use of control areas in a method for the detection of an analyte for the simultaneous detection of interferences.